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Matrix subtraction errors using repmat

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WBOZ11
WBOZ11 on 26 Sep 2018
Edited: dpb on 27 Sep 2018
I am trying to subtract a 1x1 matrix with a 10000x1. In order to have proper matrix subtraction my formula was
A=unitwt*z; % (a 1x1 matrix)
B=a; % 10000x1 matrix
AB=repmat((A,10000,1)-B).*(a 10x1 matrix)
However when doing this, I keep getting the error:
noncomformant arguments (op1 is 1x1, op2 is 10000x1).
Is there anything I am missing?

Answers (2)

OCDER
OCDER on 26 Sep 2018
Why do you need to use repmat? a 1x1 matrix is a scalar, hence you can just do A-B without repmat.
A = 2; %this is a 1x1 matrix
B = rand(10) %this is a 10x10 matrix
%these will work:
A + B
A - B
A * B
B / A
The real question is, what do you expect a 10000x1 matrix times by a 10x1 matrix to yield?
AB = (A-B) .* C
(10000x1 matrix) .* (10x1 matrix) = ?????
Or did you mean
AB = (A-B) .* C.'
(10000x1 matrix) .* (1x10 matrix) = 10000x10 matrix?

dpb
dpb on 26 Sep 2018
Edited: dpb on 27 Sep 2018
  1. You can subtract/add/multiply/divide any sized array with a scalar without repmat so you're "fixing" a problem that isn't a problem to begin with ( :) ), and
  2. You've got parentheses-nesting problem in the way you wrote the arguments to repmat if you are going to do it...
AB=(repmat(A,size(B))-B).*a_10x1_matrix;
will get as far as the subtraction, but then the matrix multiply will be non-conforming as you'll have [10000x1]*[10x1] and the inner dimensions must match. You could write the transpose there for the second if the result is intended to be [10000x10].
But, as pointed out at the top, just write
AB=(A-B).*a_10x1_matrix.';
or otherwise solve the latter problem on the end result size.
ADDENDUM
I just noticed both OCDER and I missed the 'dot' in the .* element multiply. There's an issue there in commensurate sizes as well; the problem in answering the Q? is that the intended result is ill-defined such that the intent is not known.
Explain what the output result is intended to be; perhaps illustrate the desired result with a set of small sample inputs (making up such an example might just let you see the answer to the problem on your own, even).

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